An apparatus for monitoring of the deposition of metallic films

ABSTRACT

A blank crystal is sandwiched between two electrodes in a holder in a manner such that the only contact of the electrodes with the crystal is along the outer circumference of the crystal. A preliminary evaporation of metal onto the exposed surface of the crystal establishes the desired initial resonant frequency of the crystal. A substrate is then placed in the evaporation chamber proximate the crystal and a metal film is vapor deposited on the substrate and crystal until the resonant frequency of the crystal changes by a predetermined amount. The crystal holder is designed to provide easy removal of the crystal therefrom, thereby facilitating reclamation of the crystal after metal film buildup thereon by removing the crystal from the holder and dipping the crystal in a suitable metal etchant solution.

United States Patent Sheahan et al. 1 Oct. 24, 1972 [-541 APPARATUS",FOR hm 2,479,264 8/1949 Rosenberg ..310/91 OF THE DEPOSITION OF METALLIC2,711,646 6/ 1955 Mendousse ..310/8.7

FILMS Inventors: Desmond F. Sheahan, San Carlos; George C. Callander,Palo Alto, both of Calif.

GTE Automatic Electric Laboratories Incorporated Filed: Aug. 5, 1970Appl. No.: 61,154

Assignee:

References Cited UNITED STATES PATENTS 4/ 1 95 2 Wolfskill ..1 18/95/1949 Bach ..118/9 5/1968 Unzicker et al. ..118/9 8/1956 l-lansell..310/8.3

Primary Examiner-Mervin Stein Assistant Examiner-Leo MillsteinAttorney-Kurt Mullerheim, Russel A. Cannon and Leonard R. Cool ABSTRACTA blank crystal is sandwiched between two electrodes in a holder in amanner such that the only contact of the electrodes with the crystal isalong the outer circumference of the crystal. A preliminary evaporationof metal onto the exposed surface of the crystal establishes the desiredinitial resonant frequency of the crystal. A substrate is then placed inthe evaporation chamber proximate the crystal and a metal film is vapordeposited on the substrate and crystal until the resonant frequency ofthe crystal changes by a predetermined amount. The crystal holder isdesigned to provide easy removal of the crystal therefrom, therebyfacilitating reclamation of the crystal after metal film buildup thereonby removing the crystal from the holder and dipping the crystal in asuitable metal etchant solution.

2 Claims, 4 Drawing Figures PATENTEU um 24 I972 25 FIG. 2

FIG. 4

FIG. 3

INVENTORS DESMOND F SHEAHAN BY GEORGE C, CALLANDER APPARATUS FORMONITORING or THE DEPOSITION or METALLIC FILMS BACKGROUND OF THEINVENTION Thin metallic films may be deposited on substrates by a vapordeposition technique wherein the substrate and a charge of the desiredmetal are placed in an evacuated chamber. Heat applied to the chargevaporizes the metal. The metal vapor on striking the cooler surface ofthe substrate condenses thereon and thereby builds up a thin metallicfilm. Conventional methods of monitoring the thickness and rate of thevapor deposition of such a film involve placing a crystal monitorproximate the substrate, with a crystal surface exposed whereby thechange in crystal frequency with the build up of metal deposited thereoncan be directly related to the thickness of the metal film deposited onthe substrate.

The known crystal mounting techniques and subsequent methods formonitoring metallic vapor deposition have not been completelysatisfactory because of expense, lack of accuracy, or lack ofrepeatability from one substrate to another. For example, one techniqueuses a conventional can mounted crystal. Initially the can protectingthe crystal must be removed and drilled through on each side in aposition centered on the electrode of the crystal. The can is thenreinserted over the crystal and support, and appropriately positioned tomonitor the build up of the metallic film. The difficulty with thistechnique is that the crystals must be preplated and soldered to wireleads for mounting in the can. Furthermore, in order to reclaim thecrystal for subsequent use after a given amount of metal has built up onthe crystal surface, the can unit must be disassembled to remove thecrystal therefrom so that it may be placed in a suitable solution toremove the film deposited on the surface of the crystal. Additional caremust be exercised to assure that the electrodes connected to the crystalare not damaged or possibly the electrodes will have to be unsolderedprior to placing the crystal in the cleaning solution. Also, with thistechnique the repeatability suffers since the angle at which the crystalis mounted may vary from one can unit to another.

Another technique uses a flat crystal plated on both sides which is thenmounted on a flat base plate, with the base plate forming one electrode.A spring clip makes tensioned contact with the other crystal surface andforms the other electrode. A primary difficulty with this technique anddevice is a basic instability which may arise due to heating of thecrystal and base plate which results in changes in the contact pressureof the spring clip. This in turn may cause so-called bowing of thecrystal resulting in jumps in the resonant frequency of the crystalmonitor, thus adversely affecting calibration and control of thethickness of the deposited metal. Also, this technique requirespreplated crystals.

OBJECTS AND SUMMARY OF THE INVENTION It is a principal object to providean improved apparatus for monitoring the deposition of metallic film ona substrate.

Another object of the invention is to provide an improved crystal holderfor a crystal monitor adapted to monitor the deposition of metallic filmon a substrate.

A further object of the invention is to provide an improved crystalholder of minimum complexity which keeps most of both sides of thecrystal free.

Still another object of the invention is to provide a holder for acrystal monitor which does not necessitate preplating of the crystal.

Yet another object of the invention is to provide an improved crystalmonitoring apparatus for monitoring the deposition of metallic film on asubstrate which permits inexpensive reclamation of the monitor scrystal.

According to one aspect of the invention a flat cylindrical crystal issupported on its extreme top and bottom circumferences betweenconcentric electrodes in a manner such that the majority of both crystal'faces are free of contact with other surfaces and so that one surfaceof the crystal is exposed. The crystal electrode for the non-exposedcrystal surface is the surface of an associated concentric electrodewhich is separated from the crystal surface by a small air gap over mostof its area which in effect introduces a small capacitance in serieswith the crystal.

The crystal electrode for the exposed crystal surface is completed bybuilding up on the exposed crystal surface an initial metal film byvapor deposition, for example, until a crystal resonant frequency in apredetermined range results. The crystal holder of the monitor is thenplaced proximate the substrate to be plated in a suitably evacuatedcontainer and metal is vapor deposited on the substrate and crystaluntil a predetermined change in frequency of the crystal monitorindicates that the desired thickness of metal film has been deposited onthe substrate.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a simplified representation,partially in section, of the system adapted to provide vapor depositionof a metal film on a substrate and monitor crystal;

FIG. 2 is a section view taken along line 2-2 of FIG. 1 of a portion ofa crystal holder and crystal according to the present invention;

FIG. 3 is a section view of a portion of a crystal holder and crystalaccording to the present invention; and

FIG. 4 is a schematic circuit diagram of the equivalent circuit of acrystal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For a better understanding ofthe present invention together with other and further objects andadvantages thereof, reference is made to the following description inconjunction with the above described Figures.

Referring first to FIG. 1, therein is shown a simplified system adaptedto provide vapor deposition of a metal film on a substrate. A bell jar11 rests on a platform 13. A filament 15 on which is placed a metalcharge (not shown) and the filament is connected to a suitable powersource 17. Above the filament 15 is a heat shield 16 having an openingtherein. Mounted on a support 18 is a substrate 19 and a crystal monitor21. The crystal monitor is connected by a pair of leads 45 and 47 to afrequency determining circuit 23. The crystal surface 25 in the monitor21 and the surface 27 of the substrate 19 to be plated are directedtoward the metal charged filament 15.

In operation the monitor is first calibrated to obtain an initialresonant frequency of the crystal as determined by the frequencydetermining circuit 23. Power from the source 17 is then applied to themetal charged filament 15 causing the filament to heat and give off ametal vapor. The emitted vapor migrates toward the surfaces 25 and 27 ofthe monitor crystal and substrate 19, respectively, and upon strikingthese cool surfaces condenses thereon forming a metal film. As the metalfilm builds up on the surface 25 in the monitor 21, the resonantfrequency of the monitor crystal changes, and since the monitor crystalis proximate the substrate 19, the change in crystal frequency alsoreflects the build up of the metal film on the substrate. Thus, when thefrequency of the monitor crystal changes by a predetermined amount, thedesired thickness of metal film has been deposited on the substrate, thepower is removed from the filament 15 and the substrate is removed fromthe bell jar 11.

While not absolutely necessary, the heat shield 16 does serve to preventexcessive heat from radiating toward the substrate 19 and monitor 21,thereby insuring that the monitor is subject to minimal temperaturechanges. In operation it may also be desirable to maintain the substrate19 and the monitor 21 equal distances from the filament 15 so that thethickness of metal deposited on the substrate is the same as thatdeposited on the crystal in face in the mirror.

Depending on the crystal characteristics and the thickness of metaldesired on the substrate, a single crystal may be used to control thevapor deposition of a number of consecutive substrates. However, thecrystal will eventually become non-oscillatory (or non-linear) after anexcessive build up of the metal on the exposed surface. It is thendesirable to be able to reclaim the crystal, for example, by removing itfrom the holder and dipping it in a metal etchant solution, therebypermitting repeated use of the crystal.

With a crystal and holder configuration as illustrated in FIG. 2, it ispossible to accomplish the desired abovementioned reclamation withminimal expense and relative ease. Here the crystal 31 is sandwichedbetween two electrodes 33 and 35, with the only direct contact betweenthe electrodes 33, 35 and the crystal being along the narrow outercircumference of the crystal. The outer electrode 35 is in effecthollowed outwith an inwardly protruding edge 37 at the lower endthereof. The inner surface of electrode 35 is threaded at the other endthereof. The inner electrode 33 has a protrusion 39 along its lower,outer circumference and a flat surface 41 displaced by an air space 43from the top surface of the crystal 31. Leads 45 and 47 connect theelectrodes 33 and 35, respectively, to the voltage source and frequencydetermining circuit 23. The circumference of cap 36 is threaded forengaging the threads in the end of electrode 35 and facilitating removalof the inner electrode 33 and crystal 31 therefrom.

In operation since the top electrode 33 is separated from the crystal bythe gap 43, the crystal is free to vibrate in thickness shear mode. Asillustrate in FIG. 3, during the initial stages of the metal deposition,the metal film 49 accumulates on the bottom surface of the crystal 31,forming the electrical contact between the l i "i 3.5ii1 us t r g th giii v ri i g t h e crystal oscillator as formed by the crystal mountingof the present invention; The equivalent circuit of the crystal 31 isthe capacitor 51 in series with an inductor 53 and the series LC circuitis then connected in parallel with a second capacitor 55. The effect ofthe air gap 43 of FIG. 2 is to add a small capacitance 57 in series withthe equivalent circuit of the crystal.

One system which utilizes the present invention, begins with blank ATcut quartz crystals etched to 8,000 kHz i 1 kHz. The blank crystal isinserted in the holder and the exposed surface of the crystal isbackplated by vapor deposition of the metal to about 7,940 kHz. Thecalibrated crystal in the holder is then placed proximate the substrateto be plated and the vapor deposition is commenced. As an initialapproximation it is known that the thickness of the metal deposition onthe substrate is proportional to the change in resonant frequency of thecrystal divided by the density of the metal being deposited and that therelationship generally remains linear within a frequency change of kHz200 kHz as long as the ambient temperature does not changesignificantly.

Thus it can be seen that in accordance with the present invention, nopre-plating or elaborate individual mounting of the crystal is required;the crystal may be reclaimed by simply dipping it in a metal etchantsolution; and, since the crystal faces are not in contact with othersurfaces, they are free to vibrate and are not subject to temperatureinduced contact pressure changes. Further, while there has been shownand described what is at present, considered the preferred embodiment ofthe invention, it will be recognized that changes and variations may bemade therein without departing from the spirit of the invention. It istherefore intended that the invention not be limited to the specifics ofthe foregoing description, but rather is to embrace the full scope ofthe following claims.

What is claimed is: v

1. A frequency controlling element for an oscillator circuit adapted tomonitor the thickness of vapor deposited metallic films, said elementcomprising a Atcut crystal having a resonant frequency in apredetermined range, and first and second electrode terminals supportingsaid crystal along its outer circumference with one side thereof exposedand the other side thereof separated over other than the circumferentialsupporting area from the adjacent electrode by an air gap that frees thecrystal to vibrate in its thickness shear mode, said first electrodebeing a hollow conductive sleeve having one end protruding inwardly toprovide a circumferential support member for the outer circumference ofthe exposed side of said crystal, said second electrode being formed tobe inserted within said sleeve and having a downwardly extendingcircumferential protuberance adapted to contact and support the outercircumference of the unexposed side of said crystal.

2. The invention according to claim 1 wherein said first and secondelectrode terminals are formed to permit easy withdrawal of the secondterminal from within said first terminal thereby facilitating removal.and replacement of said crystal.

1. A frequency controlling element for an oscillator circuit adapted tomonitor the thickness of vapor deposited metallic films, said elementcomprising a At-cut crystal having a resonant frequency in apredetermined range, and first and second electrode terminals supportingsaid crystal along its outer circumference with one side thereof exposedand the other side thereof separated over other than the circumferentialsupporting area from the adjacent electrode by an air gap that frees thecrystal to vibrate in its thickness shear mode, said first electrodebeing a hollow conductive sleeve having one end protruding inwardly toprovide a circumferential support member for the outer circumference ofthe exposed side of said crystal, said second electrode being formed tobe inserted within said sleeve and having a downwardly extendingcircumferential protuberance adapted to contact and support the outercircumference of the unexposed side of said crystal.
 2. The inventionaccording to claim 1 wherein said first and second electrode terminalsare formed to permit easy withdrawal of the second terminal from withinsaid first terminal thereby facilitating removal and replacement of saidcrystal.